Vigils were held outside Parliament on Thursday evening after the shock news that Cox had died after being shot and stabbed during her surgery session where she opened her office to the public to ask for help and guidance. A man has been arrested in connection with the incident and been named locally as 52-year-old Tommy Mair.

Cox had been a Labour Party MP for Batley and Spen for just over a year and worked for Oxfam before running for Parliament. She was known for her work on Syrian refugees and was a passionate human rights campaigner.

Gordon Brown and his wife Sarah on Friday issued a tribute to Cox, who had worked with Sarah Brown in the past. Gordon calls Cox "the most vivacious, personable, dynamic and committed friend you could ever have," while Sarah says in her statement she is "heartbroken" and Cox had a "truly remarkable spirit and passion."

"Our memories will be forever scarred by this moment. Our hearts will always be hurt at our country's loss. Jo Cox was the most vivacious, personable, dynamic and committed friend you could ever have. Whenever you talked to her, the compassion in her eyes and the commitment in her soul shone through. Years of public service advancing the causes she cared passionately about should have been ahead of her.

"Sarah and I were privileged to work with Jo and her husband Brendan over many years and in her tireless efforts on behalf of poor and desolate children and mothers. She she went to some of the most dangerous places in the world. The last place she should have been in danger was in her home community.

"People will say that this does not happen in Britain. This should not happen in Britain. And we must resolve that this will never again happen in Britain.

"My and Sarah's thoughts and prayers are with Brendan and their two young children Cuillin and Lejla this evening They will have all our support in the difficult days ahead.

"For them and for those of us who were Jo's friends, this is a devastating day. And it is a devastating blow to our democracy."

"Her work will go on" — Jeremy Corbyn, Labour Party leader

"We've lost a wonderful woman, we've lost a wonderful member of parliament, but our democracy will go on. Her work will go on. As we mourn her memory, we'll work in her memory to achieve that better world she spent her life trying to achieve."

"Jo died doing her public duty at the heart of our democracy, listening to and representing the people she was elected to serve."

"In the coming days, there will be questions to answer about how and why she died. But for now all our thoughts are with Jo's husband Brendan and their two young children."

"It is cruel and terrible that her life was cut short" — Hillary Clinton, presumptive US Democratic Presidential nominee

"I am horrified by the assassination of British MP Jo Cox, murdered earlier today in her district in Northern England. By all accounts, she was a rising star. It is cruel and terrible that her life was cut short by a violent act of political intolerance.

"It is critical that the United States and Britain, two of the world's oldest and greatest democracies, stand together against hatred and violence. This is how we must honor Jo Cox -- by rejecting bigotry in all its forms, and instead embracing, as she always did, everything that binds us together."

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"Our thoughts are with the people affected" — Angela Merkel, German Chancellor

German Chancellor Angela Merkel.
REUTERS/Hannibal Hanschke

"The incident is terrible, dramatic and our thoughts are with the people affected - the Labour lawmakers, the politicians. I don't want to connect this with the vote on Great Britain staying in the European Union."

"Deeply sad for Jo Cox and the British people. Through her it's our democratic ideals that were targeted. Never accept that!"

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"An assault on everybody who cares about and has faith in democracy" — John Kerry, US Secretary of State

John Kerry.
Ben Stansall/AFP

"I join you in expressing my deep sorrow that a young parliamentarian, who obviously was a young woman of enormous talent, has been killed in the conduct of her duties with her constituency. It is an assault on everybody who cares about and has faith in democracy. And our thoughts are profoundly with the family - her husband, her children - and with all of the British people, who I know feel the loss profoundly."

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"The UK is a beacon for peaceful politics" — Jeroen Dijsselbloem, Dutch Finance Minister and chair of Eurogroup of finance ministers